Hydrocarbon fluids such as engine oils, lubricants or the like are subject to conditions which cause the fluids to degrade over time. For example, oils in engines are subjected to pressures, frequent movement and heat which cause stresses to degrade the oil. Transmission fluids and other oils used in hydraulic systems are also susceptible to degradation over time. Cooking oils are another example of hydrocarbon fluids that may degrade over time.
Because of this degradation, antioxidants or antiwear agents are frequently added to hydrocarbon fluids such as oils, lubricants and the like to maintain their lubricating properties over time. For example, in using antioxidants, as long as the antioxidant remains intact, oxidation degradation of the hydrocarbon fluid is minimized so that it does not lose its desired properties.
Since using a hydrocarbon fluid beyond its useful life is undesirable, the prior art has developed various techniques and ways to monitor additives, agents or other components in hydrocarbon fluids to help determine whether the fluids still have any useful life.
In the prior art, different methods have been proposed to determine the remaining useful life of oils or the like. These methods vary in accuracy, easiness, efficiency, and economics.
In the prior art, a sensitive and simple electrochemical sensor is proposed to monitor the deterioration of a motor oil by determining the antioxidant and antiwear agent level remaining in the oil. In U.S. Pat. No. 5,518,590 to Fang, the present inventor, the electrochemical sensor is a device comprising a conductive liquid or gel-like electrolyte interphase separating the oil phase and the electrode surfaces, and is used on-line or off-line requiring no chemical or physical pretreatment of the oil.
In the prior art, cyclic voltammetry (CV) has been proposed to detect antioxidant and antiwear agents and their oxidation products in a lubricant, either on-line or off-line. U.S. Pat. Nos. 4,744,870, 4,764,258, 5,071,527 and 5,239,258, all to Kauffman, deal with this subject matter. U.S. Pat. No. 4,764,258 discloses an off-line method for evaluating a hydrocarbon fluid containing at least one additive species. An oil sample is mixed with a solvent and a solid subject to produce an analysis sample. The substrate settles from the analysis sample and then the analysis sample is placed into an electrolytic cell and subjected to voltammetric analysis. The additive may be a substituted phenol antioxidant or an oxidation product thereof. Alternatively, the additive species may be a metal-dithiodiphosphate multi-function additive or an oxidation product thereof.
In U.S. Pat. No. 5,071,527, the method of analysis includes applying an electrical potential to an electrode to produce an electrical current through the sample of oil, lubricant or fluid. The current can be produced with an extracted sample off-line or on-line with a continuously changing sample or with a sample in an on-line reservoir.
U.S. Pat. No. 4,744,870 discloses a method for measuring the remaining useful life of an ester-based lubricant containing at least one antioxidant species. A lubricant sample is mixed with a solvent, an organic base and an electrolyte to produce an analysis sample. The analysis sample is placed into an electrolytic cell and subjected to a cyclic voltammetric analysis. The current during this cyclicvoltammetric analysis is measured and recorded. Remaining useful life of the lubricant is then determined from the average maximum reduction current wave height produced.
The Kauffman patents cited above are incorporated by reference in their entirety, particularly for the purpose of identifying the types of hydrocarbon fluids and additives contemplated for use in the instant invention.
The methods using cyclic voltammetry to detect antioxidants and antiwear agents are not without their drawbacks. One skilled in the art recognizes that off-line CV wherein a solvent and an electrolyte are required works to a certain degree in determining the unreacted antioxidants such as alkylated phenols and aromatic amines. Since antioxidant and antiwear agents commonly used in the typical lubricant formulation are complicated mixtures, their signal response in CV is not well define, sensitive, or sharp and, therefore, often cannot be reproduced and accurately measured.
CV also does not work well in determining oxidation products in a lubricant due to the electrochemical inactivity of these products under conventional conditions for a CV test. On-line CV wherein electrodes are directly in contact with a lubricant, i.e., no solvent or electrolyte, does not work well since the oily components of the lubricants immediately cover the electrode surface, thereby rendering it non-functional.
In view of the drawbacks in the prior art techniques for determining the presence of additives, agents or the like in hydrocarbon fluids, particularly engine oils, a need has developed to provide an improved method and apparatus for these types of determinations.
In response to this need, the present invention provides both a method and apparatus for off-line applications which utilizes a flow injection method which can both rapidly and accurately determine or monitor the overall level of agents or additives such as antioxidants and antiwear agents in hydrocarbon fluids, particularly engine oils. While flow injection analysis is known when using converging streams of a sample and a reagent which mix and undergo chemical and physical change followed by monitoring by means of a flow-through detector, the prior art does not teach or suggest the inventive method or apparatus as described above.